A Family of Layered Phosphates Crystallizing in a Rare Geometrical Isomer of the Phosphuranylite Topology: Synthesis, Characterization, and Computational Modeling of A4[(UO2)3O2(PO4)2] (A = Alkali Metal) Exhibiting Intralayer Ion Exchange
Abstract:Single crystals of eight new layered uranyl phosphates were grown from alkali chloride fluxes: CsK[(UO)O(PO)], CsK[(UO)O(PO)], RbK[(UO)O(PO)], K[(UO)O(PO)], KNaRb[(UO)O(PO)], KNaRb[(UO)O(PO)], CsK[(UO)O(PO)], and RbK[(UO)O(PO)]. All structures crystallize in the monoclinic space group, P2/ c and contain uranyl phosphate layers with alkali metals located between the layers for charge balance. Ion exchange experiments on CsK[(UO)O(PO)], RbK[(UO)O(PO)], and K[(UO)O(PO)] demonstrated that Cs and Rb cations cannot … Show more
“…The volume of the salt is then subtracted from the overall formula unit volume of the five identical framework materials, which are then averaged. The larger formula unit volume of the pure cesium containing (Cs 2 Cs 5 F) SIM leads to a formation enthalpy that is less negative than its four counterparts; a similar trend was found in 25 , where larger alkali inclusions (and therefore V m values) resulted in less negative formation enthalpies. For the remaining SIMs of the [(UO 2 ) 3 (Si 2 O 7 ) 2 ] 6− family, both DFT and VBT predict that the chlorine containing KK 6 Cl salt is the least stable structure and the NaK 6 F salt-inclusion is the second most stable structure.…”
Section: Discussionmentioning
confidence: 55%
“…The energies include the mixing entropy of the salt-components as noted above and as was demonstrated in Juillerat et al . 25 for alkali metals. Figure 3 Lattice potential energy (U pot ) as a function of V m for SIMs, the inset shows the Ge and Si frameworks with V m between 550–750 Å 3 .…”
Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.
“…The volume of the salt is then subtracted from the overall formula unit volume of the five identical framework materials, which are then averaged. The larger formula unit volume of the pure cesium containing (Cs 2 Cs 5 F) SIM leads to a formation enthalpy that is less negative than its four counterparts; a similar trend was found in 25 , where larger alkali inclusions (and therefore V m values) resulted in less negative formation enthalpies. For the remaining SIMs of the [(UO 2 ) 3 (Si 2 O 7 ) 2 ] 6− family, both DFT and VBT predict that the chlorine containing KK 6 Cl salt is the least stable structure and the NaK 6 F salt-inclusion is the second most stable structure.…”
Section: Discussionmentioning
confidence: 55%
“…The energies include the mixing entropy of the salt-components as noted above and as was demonstrated in Juillerat et al . 25 for alkali metals. Figure 3 Lattice potential energy (U pot ) as a function of V m for SIMs, the inset shows the Ge and Si frameworks with V m between 550–750 Å 3 .…”
Formation enthalpies and Gibbs energies of actinide and rare-earth containing SIMs with silicate and germanate frameworks are reported. Volume-based thermodynamics (VBT) techniques complemented by density functional theory (DFT) were adapted and applied to these complex structures. VBT and DFT results were in closest agreement for the smaller framework silicate structure, whereas DFT in general predicts less negative enthalpies across all SIMs, regardless of framework type. Both methods predict the rare-earth silicates to be the most stable of the comparable structures calculated, with VBT results being in good agreement with the limited experimental values available from drop solution calorimetry.
“…The uranophane topology is specifically prominent among silicates, but as in the phosphuranylite topology the tetrahedrally coordinated Si or P sites can be replaced by other tetrahedrally—or even trigonal pyramidal or trigonal planar—coordinating elements. These sheet anion-topologies common among minerals have also been observed in numerous synthetic compounds including 38 belonging to the autunite, 18 belonging to the uranophane, and 16 belonging to the phosphuranylite classes (Lussier et al, 2016; Wang et al, 2017; Juillerat et al, 2018a;Juillerat and zur Loye, 2019).…”
Section: Introductionmentioning
confidence: 75%
“…There have been numerous reported uranium phosphate containing structures prepared by similar synthetic methods using UF 4 , AlPO 4 , and alkali chloride fluxes, predominantly loaded into alumina crucibles and heated at a temperature of 875°C (Juillerat et al, 2018a,b; Juillerat and zur Loye, 2019; Juillerat et al, 2019b). Cs 11 [(UO 2 ) 12 (PO 4 ) 3 O 13 ] ( 1 ) and Rb 11 [UO 2 ) 12 (PO 4 ) 3 O 12 F 2 ] ( 2 ) were first discovered as the minor product, previously unidentified red tablets, in the synthesis of Cs 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] and Rb 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ], respectively (Juillerat et al, 2018b).…”
Section: Discussionmentioning
confidence: 99%
“…Structures 3 and 4 , related to the phosphuranylite topology, were discovered when trying to optimize synthetic conditions for Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], Cs 1.7 K 4.3 [(UO 2 ) 5 O 5 (PO 4 ) 2 ], Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Rb 1.6 K 4.4 [(UO 2 ) 5 O 5 (PO 4 ) 2 ]. The synthesis of these phases is described and discussed in a recent publication which concluded that higher flux to reactant ratios (40 mmol flux to 0.5 mmol UF 4 ) favored the formation of the phases Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], while 20 mmol of flux and 0.5 mmol UF 4 lead to synthesis of Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Cs 1.7 K 4.3 [(UO 2 ) 5 O 5 (PO 4 ) 2 ] or Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Rb 1.6 K 4.4 [(UO 2 ) 5 O 5 (PO 4 ) 2 ], which could not be successfully separated (Juillerat et al, 2018a). The title phases, Cs 4.4 K 0.6 [(UO 2 ) 6 O 4 F(PO 4 ) 4 (UO 2 )] ( 3 ) and Rb 4.4 K 0.6 [(UO 2 ) 6 O 4 F(PO 4 ) 4 (UO 2 )] ( 4 ), were obtained simultaneously with Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] by further reducing the flux to reactants ratio to 10 mmol and 5 mmol of flux with 0.5 mmol UF 4 ; unfortunately, the title compounds could not be separated manually from Cs 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ] and Rb 1.4 K 2.6 [(UO 2 ) 3 O 2 (PO 4 ) 2 ], as all crystallize as yellow plates.…”
Single crystals of four new layered uranyl phosphates, including three oxyfluoride-phosphates, were synthesized by molten flux methods using alkali chloride melts, and their structures were determined by single-crystal X-ray diffraction. Cs
11
[(UO
2
)
12
(PO
4
)
3
O
13
] (
1
) and Rb
11
[UO
2
)
12
(PO
4
)
3
O
12
F
2
] (
2
) contain uranyl phosphate layers exhibiting a new sheet topology that can be related to that of β-U
3
O
8
, while Cs
4.4
K
0.6
[(UO
2
)
6
O
4
F(PO
4
)
4
(UO
2
)] (
3
) and Rb
4.4
K
0.6
[(UO
2
)
6
O
4
F(PO
4
)
4
(UO
2
)] (
4
) contain layers of a known isomer of the prominent phosphuranylite topology. The location of the fluorine in structures
2
-
4
is discussed using bond valence sums. First principles calculations were used to explore why a pure oxide structure is obtained for the Cs containing phase (
1
) and in contrast an oxyfluoride phase for the Rb containing phase (
2
). Ion exchange experiments were performed on
1
and
2
and demonstrate the ability of these structures to exchange approximately half of the parent alkali cation with a target alkali cation in an aqueous concentrated salt solution. Optical measurements were performed on
1
and
2
and the UV-vis and fluorescence spectra show features characteristic of the
uranyl group.
Four new hybrid organic‐inorganic uranyl phosphate compounds were structurally characterized after single crystals were grown via ionothermal synthesis using the ionic liquids 1‐ethyl‐3‐methylimidazolium dimethyl phosphate and 1‐ethyl‐3‐methylimidazolium dibutyl phosphate. Three of the new crystal structures presented here incorporate dimethyl or monomethyl phosphate ligands into the structural unit to form one chain‐based compound and two sheet‐based compounds. One of the structures utilizes dibutyl phosphate to form a previously reported uranyl compound, a structural polymorph that crystallized in space group P21/c in contrast to the previously reported P‐1 structure. The structural and topological relationships are compared to those of uranyl phosphate minerals as well as previously reported organophosphate compounds. The roles of the organics on the phosphate anions in impacting uranyl coordination and stabilizing the crystal structures are discussed.
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